Lewis & Short

transtrum, i, n. [root tra-, to cross; cf. trans] (cf. jugum).

  1. I. In gen., a crossbeam, transom, Vitr. 2, 1; 4, 2; 5, 1 fin.; 10, 21; Plin. 34, 12, 32, § 123 al.; cf.: transtra et tabulae navium dicuntur et tigna, quae ex pariete in parietem porriguntur, Fest. p. 367 Müll.
  2. II. Esp., a cross-bank in a vessel, a bank for rowers, a thwart.
    1. B. Sing., Cic. Verr. 2, 5, 51, § 135; Liv. Epit. 89; Pers. 5, 147.
      Plur., Caes. B. G. 3, 13; Verg. A. 4, 573; 5, 663; Ov. M. 14, 534; Luc. 3, 543.